In recent years, the development of laser is remarkable. With respect to solid lasers and semiconductor lasers which radiate infrared light having a wavelength of from 760 nm to 1,200 nm (hereinafter sometimes referred to as “infrared laser”), ones which have a high output and a small size become easily available. Even in the field of lithographic printing, such lasers are very useful as a recording light source in undergoing directly plate making of a printing plate from digital data of a computer and the like. Following this, various studies have been made regarding image forming materials which are sensitive to these various lasers.
In general, as a negative working image forming material capable of undergoing recording upon irradiation with infrared light, there are known a polymerization system image forming material having a recording layer made of a polymerizable composition containing an infrared absorber capable of absorbing energy of infrared light to convert it into heat, a radical generator capable of generating a radical by the heat obtained from the infrared absorber, and a radical polymerizable compound capable of being polymerized with the generated radical as an initiator, in which a polymerization reaction of the radical polymerizable compound is caused and advanced in an exposed area, whereby the recording layer is cured to form an image; and an acid crosslinking system image forming material having a recording layer made of an acid crosslinking composition containing an infrared absorber capable of absorbing energy of infrared light to convert it into heat, an acid generator capable of generating an acid by the heat obtained from the infrared absorber, and an acid crosslinking agent capable of being crosslinked with the generated acid as an initiator, in which a crosslinking reaction of the acid crosslinking agent is caused and advanced in an exposed area, whereby the recording layer is cured to form an image.
Such a negative working image forming material is liable to be lowered in sensitive as compared with a positive working image forming material in which solubilization of a recording layer is caused by energy of infrared light. As a result, the curing reaction of the recording layer in the exposed area became insufficient so that a problem rose in resistance to printing (press life). Accordingly, a composition which is highly sensitive to such an infrared recording light source, namely a negative working curable composition in which solubility in a developing solution is largely lowered upon exposure with infrared light, is being demanded.
As a polymerization system curable composition which is aimed to enhance the sensitivity, for example, in sensitive materials utilizing a polymerization reaction of an ethylenically unsaturated compound, there are known a sensitive material having a reduction type additive added thereto (for example, see JP-A-2002-82429) and a sensitive material having a dithio compound, etc, added thereto (for example, see JP-A-2002-90985). However, even all of these sensitive materials were not satisfactory with respect to the sensitivity in practical use. Further, such sensitive materials containing components having excellent reactivity with light or heat involved such an anxiety that the storage stability is inferior, for example, an undesired reaction is caused in handling under white light or during the preservation, or by any change of the circumferential temperature, or polymerization hindrance is caused due to oxygen in air.
On the other hand, an attempt to solve the foregoing problems has been made by controlling the structure of an infrared absorber. As such a sensitive material, there are known a sensitive material using a cyanine dye having an electron withdrawing group or a heavy atom-containing substituent on at least one of aromatic rings at both ends (for example, see JP-A-2002-278057) and a sensitive material using a cyanine dye having a specified substituent on nitrogen atoms at both ends (for example, see JP-A-2003-270781). By these sensitive materials, an enhancement of the characteristics was observed to a certain extent. However, when all of the foregoing characteristics are taken into consideration, there was still room for further improvement.
Also, in general cyanine dyes, after developing with an alkaline aqueous developing solution, the cyanine dye is liable to be deposited in the developing solution. For that reason, there was encountered a problem that the deposit adsorbed in an image area or a non-image area, thereby deteriorating the image quality of the resulting printing plate.